Applications of magnetic bearings to rotary machines are becoming more and more widespread, in particular for applications relating to natural gas and applications relating to corrosive gases.
At present, numerous applications already make use of the major advantage of a magnetic bearing whereby it can operate directly in the process gas of the machine in question, without any sealing. This is true of turboexpanders for processing natural gas, refrigerator compressors, electric motors for driving compressors, etc.
Nevertheless, there remain certain conditions to be satisfied in the event of the gas being either acid, or corrosive, or carrying particles.
Under such circumstances, it is essential to protect the coils of the bearing and the associated detector by using sealing or anti-corrosion protection technologies.
These techniques may be:                either impregnation with varnish implemented under a vacuum and under pressure so as to ensure that the assembly is practically sealed against its surroundings;        or jacketed bearings in which the stator portion of the bearing and of the detector are protected by a metal jacket made of a material that does not oxidize or corrode.        
The jacketed bearing technique has already been used industrially on compressors for storing natural gas, with a single metal jacket being implemented to protect both the bearing proper and the detector that is associated therewith.
FIG. 1 shows an example of a known jacketed radial active magnetic bearing.
A bearing armature 21 of laminated magnetic material is secured to the rotor 20 which is in contact with the process gas. Similarly, a detector armature 22 of laminated magnetic material is fitted on the rotor 20 and is likewise in contact with the process gas.
An airgap 6 having a thickness of about 0.3 millimeters (mm) to 0.5 mm separates the peripheral portion of the rotor 20 fitted with the armatures 21 and 22 from a single jacket 5 that is fitted on and secured to a common housing 11, 12, 13, 14 that incorporates both the stator of the bearing and the stator of the detector.
The stator of the bearing comprises electromagnet windings 32 associated with a yoke 31 of laminated magnetic material that presents end pole pieces in contact with the jacket 5 defining the airgap of the bearing 6.
The stator of the detector also comprises electromagnet windings 42 associated with a yoke 41 of laminated magnetic material that is in contact with the single jacket 5.
A potting compound 33, 43 may be injected into the leaktight housing 10 comprising the parts 11, 12, 13, 14 and the jacket 5 so as to embed the electromagnet windings 32, 42 in the potting compound in order to reinforce its mechanical strength.
The electromagnet windings 32, 42 of the bearing and of the detector are connected to electronic control circuits 60 that can be located outside the housing of the bearing.
The use of a single jacket 5 to define a leakproof wall both for the bearing and for the associated detector leads to making a jacket 5 of relatively great length, and that can lead to problems of differential thermal expansion between the jacket and the housing of the bearing.
Furthermore, the use of a single jacket that is common to the bearing and to the detector generally leads to problems of electrical and magnetic coupling, in particular in mean frequency ranges above 300 hertz (Hz). Such coupling has a strong negative influence on the transfer functions of systems for controlling magnetic bearings.
The common jacket 5 is made of a non-magnetic metallic material so that the inductive detector does not lose all of its sensitivity because of the presence of the protective jacket 5.
In order to withstand operating conditions (pressure, fast variations in pressure, temperature, ability to withstand corrosion and abrasion), the jacket 5 generally presents thickness lying in the range 0.3 mm to 0.5 mm, i.e. thickness similar to that of the airgap proper of the magnetic bearing.
The presence of such a jacket 5 of non-magnetic material thus amounts to increasing the thickness of the airgap of the bearing by about 100%, which leads to a significant limit on the load available from said bearing.